1. Field of the Invention
The present invention relates to a process for removing mercury contaminants from alcoholic alkali metal alkoxide solutions.
2. Discussion of the Background
The alkali metal alkoxides produced by the alkali metal amalgam process generally have an unwanted content of mercury in the ppm range. Mercury contents of 5 to 50 ppm can sometimes occur. In addition, based on analysis of these products, one reaches the conclusion that at least the majority of the mercury is present not in ionic form, but in zero-valent form (Hg.degree.).
To increase product quality and to expand the area of use of these alkoxides, it is desirable to remove mercury residue from the solutions by means of a simple and, at the same time, economical process.
One possible method for removal of mercury is provided by the known property of mercury of forming amalgams. A suitable material for this is a fine-grained material charged with a silver layer or a suitable metal having a very high surface area, such as a wool, which may be used as an electrode in an electrolysis cell. Separating off mercury by amalgam formation is performed in DE-A42 21 207 using a silver-coated fiber and in DE-A-25 18 433 using nickel wool. However, media of these types are very rapidly exhausted if the mercury content of the solution is substantial, particularly when the volume of material processed is high over a relatively long period of time. They are thus comparatively uneconomical in such cases, since recovery of the expensive amalgam-forming metals is not always technically possible.
The removal of mercury from solution by treatment with activated carbon has also been disclosed. In DE-A-34 38 098, an activated carbon pretreated with mercury or with mercury salts is used in a filter bed. According to DE-A-26 43 478, activated carbon having a high specific surface area is suitable for removal of mercury, with no statement being made regarding the particle size of the activated carbon. An activated carbon treatment is described in DE-A-20 51 725 for the brines produced in chlor-alkali electrolysis.
Cleaning of mercury-contaminated waste waters is also disclosed in JP 831128 182. In this case, use is made of a tube which is provided at both the top and bottom with an acid-resistant resin filter and which contains, between the filters, 85 to 95% by volume of activated carbon and, above this, 15 to 5% by volume of coal dust. The activated carbon in this case has particle sizes of 50% above 20 mesh and 50% below 20 mesh, while the coal dust has particle sizes of about 60 mesh (20 mesh is equivalent to approximately 840 .mu.m and 60 mesh to approximately 250 .mu.m). The contaminated waste water is passed through the tube from top to bottom, with the coal dust removing the suspended matter and the activated carbon removing the harmful substances (especially mercury). Mercury contents are reduced from 37 mg/l to 0.0005 mg/l, for example, in this process.
A shared feature of the abovementioned processes is that they are designed to remove mercury from aqueous solutions. This can be taken from the examples. Their subject-matter is, in particular, the treatment of sodium hydroxide solution, soda solutions or brines. A simple conversion to alkali metal alkoxide solutions does not prove to be possible, due in part to a lack of efficiency, due in part to insufficient service lives, and due in part to the use of non-resistant filter aids.
Even the pulverized coal prefiltration of JP 83/128 182 is not applicable to alkali metal alkoxide solutions. Pulverized coals of this type are not able to retain the fine slurry-like impurities which are present in alkali metal alkoxide solutions from chlor-alkali electrolysis. A further disadvantage in the use of an extremely fine pulverized coal is that even small amounts of suspended matter, which may be present in the alkoxide solutions to be treated, can considerably decrease the service lives due to blockage.
To remove the suspended matter and slurries, centrifuges or separators are frequently used in industry. However, equipment of this type, because of moving parts, is susceptible to breakdown and requires intensive maintenance. A method which enables suspended matter to be removed in a simpler manner is therefore wanted.